1. Field of the Invention
The present invention relates to instrumentation amplifiers, more particularly to instrumentation amplifiers for sensor signal conditioning using compensation circuits for performing temperature compensation of sensor output signals.
2. State of the Art
Sensors and transducers convert physical variables (such as pressure) into an electrical signal. Sensors are often the critical components in determining the feasibility of new products. Recent advances in silicon-based sensor have resulted in almost all new sensor applications being silicon-based. Silicon sensor technology is also displacing older technologies in traditional applications. Traditional applications of sensors include the automotive and medical industries, process and industrial controls, and military and aerospace applications. More recent applications include the consumer products market, communications, computer peripherals, product testing and factory automation.
A typical integrated-circuit pressure transducer is shown in FIG. 1 and FIG. 2. Referring to FIG. 1, diffused bridge resistors are formed on a semiconductor wafer and connected by metallization patterns in a bridge configuration. The bridge may be excited by applying across the bridge a potential E. The difference between a first potential e.sub.1 and a second potential e.sub.2 at opposite vertices of the bridge is indicative of the pressure being measured. As shown in FIG. 2, the semiconductor water is etched so as to form a pressure diaphragm. Variations in pressure cause various mounts of deflection of the pressure diaphragm.
Silicon exhibits an important electro-mechanical effect, piezoresistivity. When a silicon is strained, its resistivity changes by a reproducible amount. When pressure from a gas or liquid deflects the thin pressure diaphragm, the strain induced causes a change in electrical resistance which can be sensed by external electronics.
The electrical properties of silicon are very temperature dependent. The sensitivity of the sensor, defined as the ratio of the change in transducer output to a change in the value of the measured (e.g., pressure), therefore changes with temperature. The span, or range, of a transducer is the range of measured values over which the transducer is intended to measure, specified by upper and lower limits. FIG. 3 shows the full-scale output voltage of a typical piezoresitive transducer (PRT) as a function of temperature. As may be observed, the relation between fullscale output voltage and temperature is highly non-linear. Consequently, most devices will operate accurately only over a limited temperature range. Typically, silicon sensors require temperature compensation. By incorporating appropriate temperature-independent circuitry with the sensor, the temperature range over which sensors will operate accurately can be extended substantially.
Various techniques are known for performing span temperature compensation of PRTs. One particularly advantageous method and implementation are disclosed in U.S. patent application Ser. No. 08/435,441 entitled Method and Apparatus for Sensor Signal Conditioning Using Low-Cost, High-Accuracy Analog Circuitry (Attorney's Docket No. 018018-001), filed on even date herewith, incorporated herein by reference.
In addition to span temperature compensation, offset compensation and offset temperature compensation are also required to fully compensate the output signal of a PRT. Offset compensation and offset temperature compensation ensure that when no input is applied to the PRT the output signal indicates a zero value at any operating temperature within compensation limits. Furthermore, the sensor output signal must be amplified to a level that is usable by external circuitry. The amplifier circuitry used to perform such amplification must itself exhibit high linearity and low temperature drift; otherwise, even perfect span temperature compensation will not be sufficient to achieve acceptable results.
Considerable effort has been devoted to achieving an instrumentation amplifier having high linearity and low temperature drift. Unfortunately, such efforts have most often resulted in amplifiers that are costly to manufacture and that exhibit less-than-ideal temperature drift.
A conventional instrumentation amplifier, shown in FIG. 4, consists of a pair of buffer amplifiers 21, 23 and a differential amplifier 25. The arrangement of FIG. 4 has the disadvantage that the results of any temperature drift in the buffer amplifiers 21, 23 is only magnified in the differential amplifier 25.
In order to enable the wide-spread use of PRTs over a variety of applications, many of which are cost-driven, there is needed an instrumentation amplifier for sensor signal conditioning that exhibits high linearity and low temperature drift and that uses low-cost, high-accuracy analog circuitry.